In Turkey, we take religious education as a program called "Culture of Religion and Knowledge of Ethics". Today, When we were in class, teacher said that Prophet believed some kind of pure Abrahamic religion called "Hanif", unlike most of Meccans who were Christian, Jewish or Pagan. I've never heard such a religion and some of my classmates like me claimed that there was no such a religion. So after a loud discussion in class, I googled it and I read a article about the religion of Quraysh tribe. The article claims that even though most of the Quraysh members were Pagan, there were Christians and Jews in the tribe and Mohammad was a Jew before his prophecy. I don't think that is true (Since there are many verses in Quran that contradicts Tanakh) but I have also doubts about "Hanif" religion. Could that be an invention that was created by Arab historians who wanted to hide Mohammad's past ?
It is correct that Muhammad was not Jewish before his conversion, but ḥanīf. Being a ḥanīf essentially entailed in believing in Abrahamic monotheism (as opposed to the standard polytheism of the Arabian Peninsula at the time) but it wasn't a set religious creed. The Arabic root ḥ-n-f in ḥanīf means to incline, so ḥanīf was used in the Qurʾan to signal those who had returned to the monotheism of Abraham away from the idolatry of polytheism.
This sort of monotheism wasn't unknown. There were other figures around Muhammad's time who fell into such beliefs. One of them, the cousin of his first wife Khadija, Waraqah ibn Nawfal, was a hanīf with Christian leanings. Indeed, the fact that the Kaʿba (which became polytheistic pilgrimage center) was claimed to have been built by Abraham and his family meant that there would have been reasons for there to be ḥanīf around Mecca as a whole.
Wael Hallaq, one of the leading scholars on Islamic law, writes within his book The Origins and Evolution of Islamic Law that "Already in Mecca, Muḥammad conceived of himself as a ḥanīf, probably under the influence of a certain Zayd b. ʿAmr. Fundamentally monotheistic, ḥanīfiyya appears to have been a specifically Meccan religious development that was formed around the figure of Abraham and the Kaʿba, which he was believed to have constructed".
The existence of ḥanīf is pretty well accepted (at least within what I have read), so it would lead me to doubt that it was a later Islamic invention to hide Muhammad's Jewish roots.
Sources
Wael Hallaq, The Origins and Evolution of Islamic Law
Ira Lapidus, A History of Islamic Societies
Richard Martin et al, Encyclopedia of Islam & the Muslim World
I would also recommend that you read Reza Aslan's No God But God, which gives a decent non-Academic overview of the origins of Islam.
/u/frogbrooks gave an excellent answer detailing Mohammad's religion pre-revelation.
I will add more regarding its name using mainly the Qur'an itself as my source.
The word ( حنيف ) or ( Ḥanīf ) has been mentioned in the Qur'an 10 times in 7 different chapters, and 7 of these verses contained the word ( ابراهيم ) or ( Ibrāhīm ).
Of these 10 mentions, I picked two from Chapter 16: (Sahih International)
120. Indeed, Abraham was a [comprehensive] leader, devoutly obedient to Allah , inclining toward truth ( Ḥanīf ), and he was not of those who associate others with Allah
121. [He was] grateful for His favors. Allah chose him and guided him to a straight path
122. And We gave him good in this world, and indeed, in the Hereafter he will be among the righteous
123. Then We revealed to you, [O Muhammad], to follow the religion of Abraham, inclining toward truth ( Ḥanīf ); and he was not of those who associate with Allah.
Verse 123 says that Mohammad was ordered by God to follow the same Ḥanīfiyya which Ibrāhīm was following, which reassured Mohammad that what he was on pre-relevation is not different at all from what the Qur'an carries with it. That is the Inclination towards truth (literal translation), where complete devotion and obedience is only given to Allah alone, from verse 120.
And one from Chapter 30 : (Sahih International)
30. So direct your face toward the religion, inclining to truth. [Adhere to] the fitrah [Innateness] of Allah upon which He has created [all] people. No change should there be in the creation of Allah . That is the correct religion, but most of the people do not know.
This verse states that all the people were created with the instinct and innateness of Ḥanīfiyya. God has decreed in this verse, that if this innateness is not tampered with nor changed, it shall indeed be the correct religion. Based on that, Mohammad's innateness was not changed since the day he was born. This again reassures that he was indeed Ḥanīf and not a Jew or a Christian.
http://www.alawfa.com/Go.aspx?keyword=حنيف https://quran.com/30 https://quran.com/16
There was no seperate religion called "hanif". People who believed in Abrahamic monotheism were called hanifs, wheter they were following a specific religion, like Waraqa ibn Nawfal, or not, like Muhammad (sav).
(Ulan ateistin Türkü niye hep böyle ayri bir cesit mal oluyor? Tamam Islamiyeti sevmiyorsunuz, bari Islamiyet neyse onu sevmeyin, aklinizda kurdugunuz strawmani deyil.)